DNA replication is being studied in E. coli and in plant cells grown in suspension culture. In E. coli, the time of synthesis of key gene products is being determined using amber mutations in DNA genes in a background containing a temperature-sensitive suppressor t-RNA. By raising the temperature the synthesis of active gene product can be inhibited. In this manner, the time of synthesis can be determined, as well as the amount present in the cell. The order of synthesis of DNA genes will be determined. A gene product which regulates termination of DNA replication is being studied. A dominant allele which always terminates replication has been shown to interact with other DNA gene products and to act as an anti-mutator. Recently another mutation has been isolated which allows synthesis to continue without terminati n. This mutation has a mutator phenotype. DNA replication in plants has been studied in vivo yielding results similar to those observed in animal cells. Short pieces are first formed which then join to give longer DNA. An interesting aberration is the formation of separate strands with tandem short gaps. A powerful plant endonuclease with unique properties had been observed and partially characterized. An in vitro system of DNA replication has been developed using nuclei and deoxytriphosphates.